A common practice in the manufacture of porous substrates is to apply a coating of various materials thereto. In the manufacture of carpet material, for example, it is necessary to dye the carpet to the desired color during the manufacturing process. Also, many types of fibrous and non-fibrous webs or blankets require a binder to bond the fibers or other particles together to form a cohesive product. An example of this is an insulation pack produced by bonding together mineral fibers, such as glass fibers. A typical binder for a glass fiber insulation pack is a phenol-formaldehyde-urea binder. The binder is applied to the fibers, and, when cured, the binder enables the insulation pack to be compressed with nearly full recovery upon release of the compression.
Previously, the application of binder, dye or any other coating material to porous substrates, such as carpets or mineral fiber webs, has been by one of several methods. The common method in the manufacture of mineral fiber packs is to spray a binder material or coating material onto the fibers prior to collecting the fibers in the form of an insulation pack. This process has certain deficiencies in that the binder has to be applied in a hot fiber-forming zone, thereby creating air pollution problems. Also, binder application onto air-borne fibers is inherently non-uniform. Another process for applying coatings to porous substrates is that of transporting the substrate through a liquid bath, such as is used to dye fabrics, including carpets. This process is deficient in that a large percentage of water or other carrier medium remains in the porous substrate after the coating process, and must be removed by costly methods, such as by drying ovens. Also, liquid bath applicators provide no control of penetration of the liquid into the substrate.
Another method for coating porous substrates is that of creating a foam containing the coating material, such as the binder or the dye, and impregnating the porous material with the foam. The use of the foam material facilitates a uniform coating on all the material of the substrate, and applies the coating with a minimum amount of carrier medium, such as water. Typically, the foams are applied as a layer to the substrate, and caused to impregnate the substrate by the use of a doctor blade. A process for forcing a layer of foam into a porous substrate is disclosed in U.S. Pat. No. 4,188,355, to Graham et al., which provides for a suction apparatus to force the binder foam into the insulation pack. The use of a suction device to force the foam into the pack is not entirely satisfactory, however, and is made more difficult by the inherent difficulty in transporting a fragile pack of fibers through a narrow opening and past a foam applicator. Also, suction devices are limited in not being able to produce any pressures higher than one atmosphere. There is a need for a method and apparatus for applying foam to a porous substrate in the absence of a vacuum apparatus, which is inherently pressure limited.